1. Field of the Invention
This invention relates to ion implantation and in particular to a method of modifying a ribbon-shaped ion beam which has an elongate cross-section normal to a beam direction, and also to apparatus for modifying such a ribbon-shaped ion beam.
2. Background Information
Ribbon-shaped ion beams are known to be used for implanting ions into substrates, particularly semiconductor substrates as used in the semiconductor manufacturing industry. Typically, such semiconductor substrates comprise relatively thin wafers of the substrate, commonly formed of monocrystalline silicon. Wafers of monocrystalline silicon may be circular with a diameter of about 200 or about 300 mm, although other shapes and dimensions may be used. A ribbon-shaped ion beam can be used for implanting ions into such wafers, in which case the ribbon-shaped ion beam may be controlled to have an elongate cross-section at the location of the wafer to be implanted which has a length just greater than the diameter of the wafer. Then, in order to ensure an even implantation of ions over the entire surface of the wafer, it may be necessary only to produce relative movement between the ribbon-shaped ion beam and the wafer in a direction perpendicular to the elongate cross-section direction of the ion beam.
It is a known requirement in the field of ion implantation into semiconductor substrates, that the dose of ions being implanted should be delivered evenly over the entire surface of the wafer. To achieve this, it is therefore important that the ribbon-shaped ion beam has an even distribution of intensity over the width of the elongate cross-section of the ion beam at the location of the wafer being implanted. Intensity non-uniformity over the elongate cross-section of the ribbon beam can be caused at the ion source, through a number of processes, including errors in the alignment of extraction electrodes, temperature effects, and physical changes causing plasma non-uniformity within the ion source chamber. It is therefore become a common practice in the production of ribbon-shaped beams for ion implantation, to employ a beam line profilometer, which can measure intensity variations across the ribbon beam, in combination with some suitable uniformity correction apparatus.
A known apparatus for modifying a ribbon-shaped ion beam has a high magnetic permeability rectangular steel structure defining an elongate open space which accommodates the ribbon-shaped beam passing through the rectangular structure with the length of the elongate open space aligned with the long cross-section of the ribbon beam. Multiple individual coils are distributed along each of the long bars of the rectangular steel structure, with individual controllable power supplies establishing current through each of the coils. These multiple coils may be energized to produce selected field distributions in the open space within the rectangular steel structure, including quadrupole, other multipole field distributions and other useful magnetic field distributions.
It is also known to employ quadrupole and other multipole field distributions in the field of magnetic spectography, for correcting aberrations in the trajectories of particles travelling through a spectrograph. The technical considerations for magnetic spectrographs are, however, very different compared to those applicable for a ribbon-shaped beam used in an ion implanter. Most particularly, space charge considerations are normally completely absent in magnetic spectrograph applications, as there is seldom more than a single charged particle traversing any region of a spectrograph at any time. In general, the considerations relating to the correction of optical aberrations along the charged particle path of a magnetic spectrograph are quite different from the considerations mentioned above for modifying, and improving the uniformity, of a ribbon-shaped ion beam in the ion implantation of semiconductor substrates.